Recently, wireless data, entertainment and mobile communications technologies have become increasingly prevalent, particularly in the household environment. The convergence of these wireless data, entertainment and mobile communications within the home and elsewhere has created the need for merging many disparate devices into a single wireless network architecture capable of seamlessly supporting and integrating the requirements of all of these devices. Seamless connectivity and rapid transfer of data, without confusing cables and wires for various interfaces that will not and cannot talk to each other, is a compelling proposition for a broad market.
To that end, communication industry consortia such as the MultiBand OFDM Alliance (MBOA), Digital Living Network Alliance (DLNA) and the WiMedia Alliance are establishing design guidelines and standards to ensure interoperability of these wireless devices. For example, Wireless 1394, Wireless USB, and native IP-based applications are currently under development based on Ultrawideband (UWB) radio or WiMedia Convergence Platform.
Although it began as a military application dating from the 1960s, UWB has recently been utilized as a high data rate (480+Mbps), short-range (up to 20 meters) technology that is well suited to emerging applications in the consumer electronics, personal computing and mobile markets. When compared to other existing and nascent technologies capable wireless connectivity, the performance benefits of UWB are compelling. For example, transferring a 1 Gbyte file full of vacation pictures from a digital camera to a computer take merely seconds with UWB compared to hours using other currently available, technologies (i.e. Bluetooth) and consume far less battery power in doing so.
Typically, devices which employ UWB utilize a fixed channel bandwidth that is static in frequency, or a fixed channel bandwidth that can be frequency agile. In either case, the bandwidth utilized by a device must remain substantially fixed. Thus, the range and data rate of the device is, for the most part, determined by the modulation/coding of the signal, and the power with which the signal is transmitted.
In most cases as UWB, by definition, is spread over a broad spectral range, the power spectral density of a signal utilized by a UWB device is usually very low, and hence, usually results in low incidence of interference with other systems which may be utilizing the same bandwidth as the UWB device or system.
Power spectral density, however, is a function of distance. Consequently, if a UWB device was in close proximity to another wireless system there is a significant potential for interference between the UWB device and the wireless system.
Additionally, there may be frequency bands within a UWB channel where it is important to suppress interference. For example, some existing UWB spectrum allocation encompasses the C-Band satellite downlinks. Thus, there is a potential for UWB systems to interfere with television reception of those types of system.
As can be seen then, being able to control the shape and energy of a UWB signal is important for a myriad number of reasons, including regulatory, commercial and interference issues. One approach to deal with these types of issues is to employ a pulse shaping filter to the UWB waveform prior to transmission or reception of a UWB signal. Utilizing a pulse shaping filter, interference or overlap between the bandwidths of the UWB devices and a wireless system may be minimized.
Various types of digital notch filters have typically been applied to a signal to shape the power spectrum of the signal. In most cases, these digital notch filters are finite impulse response filters of several bits. These types of digital notch filters, however, are highly problematic in real-world applications as they tend to be highly disruptive, distorting the pulse shape of a transmitted UWB signal, introducing ringing and interchip interference and hence destroying information contained in the pulse shape while making the signal harder to decode. These types of disruptions occur mainly because of spectral loss and time dispersion side effects of the finite impulse response filters. In addition, certain types of these pulse shaping filters may add cost and complexity.
Statistical filters, on the other hand, may be used to introduce correlation to a bit stream that doesn't distort the resulting pulse shape of a signal, but that changes the power spectrum of that signal. Additionally, a key aspect of a statistical filter such as this is that it may have a smooth roll-off in a transition band and the slope in the transition band may be relatively low. Unfortunately, due to the mathematical complexity of statistical filter, equation based design of these filters is difficult. More specifically, traditionally it has been difficult to design a statistical filter to meet the specification of a given power spectrum. While various sorts of line coding techniques have been experimented with, no systematic method has been demonstrated in the literature that can shape a line code to fit an arbitrary spectral shape.
Thus, as can be seen, there is a need for methods and systems for the design and implementation of statistical filters in conjunction with the shaping of a power spectrum of a signal.